1. Field of the Invention
The present invention relates to a method and apparatus for representing ultrasonic echo signals arriving in polar coordinates obtained by means of a sector scan technique, and in particular to such a method and apparatus wherein the echo signals are intermediately stored in a two-dimensional image memory and are read out therefrom for the purpose of representation on a monitor conforming to television standards, wherein the two-dimensional image memory contains a memory matrix for accommodating a complete ultrasonic echo image.
2. Description of the Prior Art
In conventional ultrasonic sector scan procedures, it is desirable to display the image information, produced in the form of vectors, as a homogeneous image on the display screen of a television monitor. As used herein the term "vector" means the echo information received along an ultrasonic line after emission of an ultrasonic pulse. For various scan times, in the sector scan technique, various angular directions proceeding from a common point of origin for the respective vectors result. Thus for a real time display conforming to television standards for the purpose of ultrasonic technology, it is necessary to employ a scan converter with an intermediate memory. A scan converter converts the ultrasonic scan into a display signal conforming to television standards which can be represented on the monitor.
In order to represent ultrasonic sectional images according to television standards, it is thus necessary to intermediately store the generated images in one or more intermediate memories prior to representation on the television monitor. Generally a single two-dimensional image memory in conjunction with a one-dimensional vector memory or vertical memory are employed, as well as a one-directional horizontal memory or television line memory connected after the image memory in the direction of information flow. The vector memory and the horizontal memory are generally each operated according to the alternation buffer procedure.
For such a memory procedure, which employs a two-dimensional image memory having a matrix equipped to accept a complete ultrasonic echo image, a disadvantage is that the video image displayed on the monitor results from at least two chronologically successively disposed ultrasonic scans. This means that movements of the examined subject are detected at different times and therefore the subject, or parts thereof, according to their movement, may be represented at a different location on the display screen. For example, the human heart or other organs in the medical field may move back and forth on the display screen as a result. A further disadvantage is that the sickle-shaped dividing line which separates the new image from the preceeding image is normally not perceived on the display screen. Erroneous interpretation may thus occur in the assessment or evaluation of a moved process or a subject which is given to movement. This is a considerable disadvantage, particularly in the medical field.
The above-described memory operations are customarily carried out utilizing digital semiconductor memories. A scan converter for an ultrasonic sector scanner is described, for example, in U.S. Pat. No. 4,245,250 having the objective of attaining as high a frame repetition rate (or frame frequency) as possible. The scan lines generated by the sector scanner (phased array) are scanned with varying frequency. The ultrasonic echo signals or scan data thus acquired along a scan line are written into one of four main memories. Pre-connected time delay devices serve as intermediate memories. Each of the main memories, which are two-dimensional memories, is equipped to accept one fourth of the ultrasonic image. Through application of the variable frequency during the scanning, a data reduction is effective during entry of the information into the main memories.